Because currently you can write something like:
```python
KwargInfo('capture', bool)
```
Which proclaims "this must be bool", but the default is then not valid.
Because of the convertor function we have no guarantee that what we're
getting is in fact a `Dict[str, TYPE_var]`, it might well be anything in
the values, so we need to do some casting and set the return type to
object. This works out fine in practice as our declared `TypeDict`
inputs in the actual function signatures will be used instead.
This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.
This allows checking specific values that are added or deprecated, which
we do a surprising amount of. This works with both containers and scalar
values
Xavier Claessens
Dylan Baker
Daniel Mensinger